This proposal examineshow children and adults develop understandings of the world around them given clear evidence of the incompleteness of their knowledge. This knowledge, known here as "folkscience", is explored through three strands: tracking of casual structure, evaluating the quality of one's own knowledge and that of others, and having the ability to grasp the social infrastructure that supports the partial knowledge in any one mind. Tracking of causal structure will be explored by conducting studies on 1. the conditions under which young children attribute changes in regular events to animmate agents., 2. the ways in which children and adults overextend essentialism and related causal mechanisms to inessential categories, 3. the limitations of Bayes nets as models of partial causal representations 4. how the structure of causal chains influences perceived quality of explanations and 5. how different cognitive "stances" emerge and interact.. The evaluation of knowledge will be examined through studies that 1. uncover mechanisms for 'illusions of explanatory depth" and for "illusions of insight", 2. examine how children and adults understand what makes a good vs. bad summary of an initially elaborate explanation, 3. examine the basis for udgments of the relative complexity of explanations and the ways in which suchjudgments influence deference and the seeking of expertise, and 4. examine how children evaluate the legitimacy of areas of explanatory expertise. Studies on grasping the social infrastructure will look at 1. how children infer causally mportant features from intentional actions of others, 2. how children learn to distinguish expertise on biological phenomena from psychological phenomena, 3. how notions of the divison of cognitive labor might vary across children and adults from different groups and backgrounds, 4. how children understand the need o shift patterns of knowledge deference in different contexts, and 5. how and why children underestimate he extent to which one's own knowledge is dependent on knowledge in other minds.